The present invention relates to an accumulator provided with an insert that can be equipped in a hydraulic circuit or the like, and more particularly to an accumulator provided with an insert to be used for absorbing pressure pulsation of a pump.
Heretofore, a bladder type accumulator has been used as an accumulator for absorbing pressure pulsation of a pump, and in such an accumulator an interior of a vessel main body provided with a feed/discharge port or port is partitioned into a gas chamber and a liquid chamber by means of a bladder formed of an elastic member, gas held at a predetermined pressure is filled in the gas chamber, while the liquid chamber is communicated with a hydraulic circuit, and liquid is made to flow into and out of the liquid chamber through communication holes in a liquid chamber wall.
In the case of a bladder type accumulator in the prior art, if it is used for an oil hydraulic pump accompanied by pressure pulsation of several handred cycles such as, for instance, a gear pump, a vane pump or the like, it cannot sufficient absorb the pressure pulsation.
That is, a bladder type accumulator in the prior art could not absorb pump pressure pulsation at a high frequency, and the reason is as follows.
In FIG. 4, reference character A designates a piping circuit connecting a pump B and a tank C, reference character D designates a neck of an accumulator ACC connected in the piping circuit A, reference character E designates a bladder which partitions a gas chamber F and a liquid chamber G from each other, and reference character H designates a choke formed in the piping circuit A.
In order to absorb pump pressure pulsation at a high frequency by means of the accumulator ACC, two conditions are considered to be necessary, that is, condition --(a) that a natural frequency of the accumulator system should be made to be close to a pulsating frequency of the pump, and condition-(b) that a frequency range of attenuation for pressure pulsation should be chosen to be broad so as to attenuate two or three pressure pulsation frequency components, are considered to be necessary.
In connection to condition--(b) above, a mass M which governs a natural frequency of an accumulator is represented, as is well known, approximately by the following formula, as viewed from the neck D: EQU M=.rho.SL,
where .rho. represents a density of liquid, S represents a cross-section area of a neck, that is, a cross-section of an access port for a liquid chamber, and L represents an effective length of the neck, that is, a length passed by the liquid after it has entered the liquid chamber before it collides with the bladder.
A spring constant K of the filled gas in the accumulator ACC as viewed from the neck D is represented, as is well known, by the folowing formula: ##EQU1## where r represents a polytropic number of the filled gas, P represents an inherent average pressure and V represents a volume of the gas at the pressure P. If a natural frequency fn of the accumulator is calculated on the basis of the above formulae, it is represented by the folowing formula: ##EQU2## From this formula it is seen that in order to increase the natural frequency fn of the accumulator, in the event that P and .rho. are respectively held constant, it is only necessary to increase the neck cross-section area S, to decrease the neck effective length L and to decrease the volume V of the gas at the pressure P.
Hence, it will be conceived to decrease the volume V of the gas by reducing a diameter of the bladder, but if the diameter is reduced, a surface area of the bladder become small and this is disadvantageous for effectively absorbing pressure pulsation.